As has been disclosed, for example, in the Japanese Patent Publication (unexamined) 2000-55947 and the Japanese Patent Publication (unexamined) 2002-171690 respectively, according to the publicly known instantaneous voltage dip detection devices, results of subtraction between an absolute value waveform of a reference sine wave and a reference cosine wave each synchronizing with a supply voltage and an absolute value waveform of a supply sine wave and a supply cosine wave are respectively obtained; the obtained results of subtraction are then integrated; and in the case where any of the results of integration exceeds a reference value, it is determined that a voltage dip has occurred.
In the aforementioned conventional system, however, since the integration is utilized for the detection of voltage dip, a problem exists in that it takes a time of about ¼ cycle of an AC waveform in the detection and determination. Moreover, another problem exists in that if arranging an instantaneous voltage dip compensating device using such an instantaneous voltage dip detection device, service interruption over ¼ cycle takes place from the start of voltage dip up to the switching to the voltage dip compensating operation.